With the arrival of new-generation satellites, the Satcom satellite communications means operating in the X/Ka bands ought to be generalized to miniaturized terminals. With this in mind, it is therefore necessary to be able to provide services of GSM (Global System For Mobile communication) type i.e., for example, voice services, short message exchanges (SMS), file transfers or BFT service, to these very lightweight and mobile stations known by the expression “Ultra Light On The Move”.
A Satcom network must serve numerous terminals, generally at least a hundred. These terminals can be fixed terminals, stations with variable capabilities, mobile terminals known by the abbreviation OTM (“On The Move”), comprising small antennas and a minimum need for connectedness, OTP (“On The Pause”) terminals with a significant bitrate need for carrying out data repatriation operations, for example. A Satcom terminal can have very variable needs in terms of bitrate: a few 100 bits/s for “BFT” file transfers, a few kbits/s for Voice over IP or VoIP, a few kbits/s for a communication node, a few 100 kbits/s for data and video. These requirements pose certain technical problems. In the case of large bitrates, very low bitrate carriers are not compatible with OTM applications. When there is a need to send short messages regularly, of SMS or BFT type, the services request few resources, but periodically. Other applications demand a certain flexibility in the use of the resources, adaptation of the carriers (numbers, frequencies, coding, modulation) as a function of the number of terminals and of the services requested (BFT/SMS, VoIP, Data/Video). For all systems, a need to resist interference also exists, especially when dealing with small terminals.
The state of the art describes various systems. It is known to propose BFT file transfer and SMS transmission services, but only in the L band, nothing exists in the X/Ku/Ka band and the problem of data protection is not raised. This solution is not appropriate for a wider communication system. In the X/Ku/Ka bands several solutions exist, but they offer neither protection, nor flexibility in the use of the spatial resources. The DVB-RCS (Digital Video Broadcasting-Return Channel System) waveforms in MultiFrequency Time Division Multiple Access (“MF-TDMA”) mode and in Single Channel Per Carrier SCPC (DVB-S2) support mode are different in terms of coding/modulation; their implementation in general requires two different modems. In the case of the SCPC/MF-TDMA hybrid mode, the transition from one mode to the other therefore requires a rebooting of the modem, thereby engendering a loss of transmission and of service for 1 to 2 mins, for example.
The conventional MF-TDMA systems require complex planning involving a large number of parameters such as the number of stations, their bitrates, their availability. This planning is in general sub-optimal and this is all the truer for networks whose number of stations and whose IP Internet needs are very variable over time. The number of carriers must be adapted to the number of stations in operation. If too large a number of carriers are defined, then they will be under-used when there are few stations, and conversely, if there are few carriers, they will be sub-optimal in relation to the IP Internet needs and the propagation conditions of a large number of stations. Moreover, these systems offer only limited protection since they do not offer any means of securing the communications, for example they do not use the mechanism of frequency evasion.
The existing prior art solutions known to the applicant do not offer any medium bitrate/high bitrate continuity of services, do not allow flexibility in the use of the resources, do not cover the very low bitrates and do not offer the possibility of protecting the data during communications.
The following abbreviations will be used in the subsequent description:
ACM: Adaptive Coding and Modulation, technique which consists in dynamically adapting the coding rate and the order of the modulation used as a function of the quality of the received signal,
PIM: message equivalent to an SMS (Short Message System) short message,
TDMA: Time Division Multiple Access,
SCPC: Single Channel Per Carrier.
The term “station” or “terminal” refers to one and the same device.